1. FIELD OF THE INVENTION
The present invention relates to a drive wheel torque controlling system for a vehicle, comprising controlling means for controlling a variable differential motion limiting device provided between both drive wheels to distribute a predetermined LSD torque from a racing or spinning drive wheel to a non-racing or non-spinning drive wheel.
2. DESCRIPTION OF THE RELATED ART
During travelling of a vehicle on a so-called "split u" road with different friction coefficients on left and right road surfaces, when a drive wheel on the road surface having a lower .mu. slips, the differential motion of the drive wheels is limited by a differential motion limiting device to distribute a "limited slip differential" torque (which will be referred to as "LSD torque" hereinafter) from the drive wheel on the lower .mu. road surface to the drive wheel on the higher .mu. road surface to transmit the driving force of the drive wheel on the higher .mu. road surface to the road surface, thereby enhancing the running performance on the split .mu. road.
There are conventionally known drive wheel torque controlling systems in a vehicle equipped with such differential motion limiting device, wherein the compatibility between the suppression of racing or spinning of drive wheels and the running property on a split .mu. road is provided by reducing the output from an engine when the slip ratio of the drive wheel exceeds a predetermined value (see Japanese Patent Application Laid-open No. 145241/89). In the system described in the above publication, when calculating the slip ratio of the drive wheels, a smaller one of wheel speeds of the left and right drive wheels is selected as a drive wheel speed.
In a vehicle in which a variable differential motion limiting device for distributing the LSD torque from the racing drive wheel to the non-racing drive wheel is electronically controlled, if the amount of LSD torque distributed is calculated on the basis of the smaller one of the left and right drive wheel speeds in the above manner, when an outer-side drive wheel rides on a low .mu. road, for example, during turning movement of the vehicle, the LSD torque may be excessively distributed to an inner-side drive wheel, promoting vehicle instability. In addition, when one of the drive wheels rides on the low .mu. road to race, for example, in a higher speed region, the LSD torque may be excessively distributed to the other drive wheel, increasing the difficulty in satisfactorily controlling the behavior of the vehicle.